Complications

In particular, I feel like there's a lot of potential in something along the lines of the Apocalypse World savvyhead workspace, which isn't actually rolled at all. The player tells the MC what they want to build, and then the MC gives them some requirements or complications, things like needing someone specific's help, needing to take something apart to build it, or it being inherently dangerous. Things, specifically, that could lead to plot developments: get the players to talk to NPCs that have connections to other stuff, get the PCs to interact more if they've been doing separate stuff, get them to go somewhere where they'll learn something. Stuff like that seems like a cool way of having powers add depth and complications to the story even as they solve problems. I could see something in the general Chuubo's or Nobilis direction where you have awesome cosmic powers that are guaranteed to work, but there will be narrative costs that vary, or even are negotiated between the HG and the player in some way. (Some Chuubo's powers do have restrictions or side-effects, of course, but often they're more consistent limitations and less open-ended costs.)

This type of approach is a bit like a Chuubo's quest miracle, in that there's some HG-specified checkboxes you need to do to accomplish things. I'd expect this sort of narratively-costed power to generally take less time and serve a different role in the story. What I’m going for is less a long-term project, and more “you can clearly use this power to solve this problem, but I as the HG am going to make it interesting by making you jump through some hoops that tie it to the other PCs/stuff going on in the story/otherwise have it make your life more interesting”. Don’t just solve problems, add complications.

(These are both different from Chuubo's "be careful what you wish for" wish effects, which can have an unexpected narrative complication after the fact as opposed to part of the setup of something.)

My other basis point for this sort of thing is Dresden Files. In the Dresden Files RPG, spellcasting rituals are player-driven and similar in some ways to a Chuubo's ritual. The player might need to do some number of preparation "mini-scenes" to work up a proper ritual, but is presented as player-driven creativity rather than complications. (Which can be cool too, but isn't what I'm going for here.) What happens sometimes in the Dresden Files books or in, like, Buffy the Vampire Slayer is that they're researching some spell and they find, arglefraster, that they need some specific thing that's both nontrivial to get and narratively interesting. That's the sort of complication I'd be thinking about here. You can have it basically be the case that magic can solve anything, or whatever, without that making it boring or only limited by MP (or equivalent), because while it's solving problems it creates complications that are interesting problems in their own right. And giving the GM at least partial control over these complications can help them steer different plot threads together and otherwise improve the narrative.

I feel like there's a lot of potential for complications to come back to the feel of some of the Apocalypse World MC moves: when players want to do something, you "Tell them the possible consequences and ask," or "Offer an opportunity, with or without a cost.") When they get a partial success doing something under fire: “the MC can offer you a worse outcome, a hard bargain, or an ugly choice”. It's not about saying no, or about coming up with an obstacle so onerous that it's like saying know. It's about making the choices the player has interesting, which involves both being open about what the choices mean and also having the options connect to what they care about.

One of these days I'm going to have to turn some of these musings into an actual attempt at a game. Until then, I'll stick to my Chuubo's game, where I'm ending up with some wonderful complications without any particular rules support.